|tokell [at] fmrib.ox.ac.uk|
|Tel||+44 (0) 1865 610476|
Vascular Territory Map
Three axial slices through different locations in the brain. Colour shows the origin of the blood signal (red = right internal carotid artery, green = left internal carotid artery, blue = right vertebral artery, magenta = left vertebral artery)
A simulation of blood flowing into the brain and through the circle of Willis in axial (top row) and coronal (bottom row) views.
Vascular territories above the circle of Willis
Here the colours show vascular territories of a larger number of arteries tagged above the circle of Willis.
University Research Lecturer
- Royal Academy of Engineering Research Fellow
- Head of Neurovascular Imaging
My research focusses on the development of novel non-invasive MRI methods which visualise blood flowing through the arteries that feed the brain and the resulting perfusion of the brain tissue. In particular, I am interested in techniques which allow blood from individual feeding arteries to be followed through the vascular tree. In addition to providing information about the structural and functional status of each artery, these methods allows the assessment of "collateral blood flow". This is when the main feeding artery to a certain brain region becomes blocked or significantly narrowed, but the flow of blood from secondary arteries maintains perfusion in this brain region, preventing a significant stroke. The presence or absence of collateral flow can be important in deciding between potential treatment options in patients with arterial disease. These vessel-selective strategies also have applications in diseases where the arterial source of blood flow is important, such as tumours and arteriovenous malformations.
My work is mainly based on a method known as "vessel-encoded arterial spin labelling". This allows blood in various combinations of arteries to be magnetically labelled. Acquiring a series of images with different combinations of tagged arteries allows the signal arising from each artery to be separated in post-processing whilst maintaining a high signal-to-noise ratio. I have applied this method to perform vascular territory mapping, in which the perfusion territories of the main feeding arteries to the brain are visualised. In addition, I have extended this approach to dynamically visualise blood flowing through the arteries feeding the brain, allowing assessment of arterial disease directly. I have also developed post-processing methods to extract quantitative blood flow rates from these images, which will enable a more objective assessment of each patient to be made.
I am currently trialling these new methods in collaboration with clinical colleagues in a range of patient groups, including those with chronic arterial disease, acute stroke and arteriovenous malformation. I hope that this will show the potential utility of these techniques for understanding the progression of these diseases, and in the longer term help with diagnosis, prognosis and therapeutic planning in these patients.
I have recently been awarded a five year research fellowship from the Royal Academy of Engineering, which provides me with an excellent opportunity to take forward this research. In this project, I aim to address the key downside to these imaging techniques, which is that obtaining 3D time-resolved images of the arteries as well as maps of tissue perfusion is time-consuming, and therefore difficult to apply in a clinical setting. I plan to design a single scan which can track the flow of blood through the arteries, all the way into the tissue, thereby providing both sets of information at the same time. I will use recently developed methods to accelerate this process, allowing images to be acquired in a fraction of the time normally required. I will also adapt mathematical models to allow a range of quantitative physiological information to be extracted from the images, before testing this new technique in a range of different patient groups.
Optimization of 4D vessel-selective arterial spin labeling angiography using balanced steady-state free precession and vessel-encoding.
Okell TW. et al, (2016), NMR Biomed, 29, 776 - 786
An Optimized Encoding Scheme for Planning Vessel-Encoded Pseudocontinuous Arterial Spin Labeling.
Berry ESK. et al, (2015), Magn Reson Med, 74, 1248 - 1256
Cerebral blood flow quantification using vessel-encoded arterial spin labeling.
Okell TW. et al, (2013), J Cereb Blood Flow Metab, 33, 1716 - 1724
A theoretical framework for quantifying blood volume flow rate from dynamic angiographic data and application to vessel-encoded arterial spin labeling MRI.
Okell TW. et al, (2013), Med Image Anal, 17, 1025 - 1036
Vessel-encoded dynamic magnetic resonance angiography using arterial spin labeling.
Okell TW. et al, (2010), Magn Reson Med, 64, 698 - 706
A systematic study of the sensitivity of partial volume correction methods for the quantification of perfusion from pseudo-continuous arterial spin labeling MRI.
Zhao MY. et al, (2017), Neuroimage, 162, 384 - 397
Relationship between collateral blood flow and hemodynamic impairment in carotid artery disease
Hartkamp N. et al, (2017), Journal of Cerebral Blood Flow and Metabolism
Feasibility of Flat Panel Detector CT in Perfusion Assessment of Brain Arteriovenous Malformations: Initial Clinical Experience.
Garcia M. et al, (2017), AJNR Am J Neuroradiol, 38, 735 - 739
Structural Connectivity Variances Underlie Functional and Behavioral Changes During Pain Relief Induced by Neuromodulation.
Lin RL. et al, (2017), Sci Rep, 7
Consensus statement on current and emerging methods for the diagnosis and evaluation of cerebrovascular disease.
Donahue MJ. et al, (2017), J Cereb Blood Flow Metab
Dynamic angiography above the circle of Willis
- Neurovascular Imaging Research Group